Digital multifunction reprographic systems are well known and have replaced optical reprographic systems as a way to reproduce images. In these conventional digital multifunction reprographic systems, a scanner accepts a document to be copied and converts the document into electronic image(s). These images, usually in the form of pages, are then passed to a central control unit which may re-order or reorganize these pages and then, depending on the request of the user of the device, send the pages or images to a destination. Often this destination is an attached printing unit which makes one or more copies of the original document.
However, these conventional devices perform many other functions besides simple copying. The central control unit is usually equipped with a combination of hardware and software elements that enable it to accept input from other sources. The other sources may include some sort of network interface and/or an interface to a telephone system to enable FAX input.
The network interface is usually configured so that it can accept jobs to be printed from any computer source that is connected to the network. This configuration normally includes elements that can convert input documents formatted in one or more page description languages (PDLs) to the native format of the printing device.
An important inner component of such a conventional multifunction digital device is the image path. This is the combination of software and hardware elements that accepts the electronic images from a multiplicity of sources and performs any operations needed to convert the images to the format desired for the various output paths. The image path is usually one of the more complex and costly components of such digital multifunction devices.
The image path for a conventional multifunction device usually has several constraints. One the hand, there is a desire to make the image path utilize data in a multi-bit per pixel format so as to provide for maximum image quality and a minimum loss of critical information in the transformation of documents from paper to electronic form. On the other hand, there are cost constraints and performance limits on the devices or software that comprise the image path.
Conventional image path electronics may also utilize binary image paths. In this situation, if the input information is scanned in a binary manner at sufficiently high resolution, the scanned image can be reconstructed at the output with little or no perceptible loss of image quality.
Another component of many conventional multifunction devices, especially for those devices having a printing engine that is capable of producing colored output, is the use of analog modulation schemes for the output. In these devices, analog data, in the form of multi-bit pixels, is presented to the modulator of the output printing device. The modulator compares the analog equivalent of the input byte of data to a periodic saw tooth wave. The output therefrom is a signal to the laser imaging component that is pulsewidth modulated by the data stream.
One recent development for conventional multifunction reprographic machines is the use of both binary and analog data in the image path. In such a hybrid image path, the data from the scanner is digitized and converted to binary. All of the intermediate elements of the image path are designed to work with the compact binary data format. Only at the output is the data converted to multi-bit analog form.
One way to implement the resolution conversion is to pass the binary data through the digital equivalent of a two-dimensional low pass filter. The low pass filter may replace each pixel in the binary image by the average of the values within some window centered on the pixel of interest. While such a system does an adequate job of converting the high resolution binary data to analog data, these solutions also have the deleterious effect of smearing sharp edges in the original document. Such an effect is particularly detrimental when reproducing text and line art.
A desirable modification to hybrid image paths would be a system wherein the conversion from binary format to analog format could take into account the existence of sharp edges in the image. Ideally, such a system would be adaptive, that is, the system would change its behavior so that it would apply a resolution conversion process appropriate to sharp edges for those parts of the image that have such edges, but use a different process that was better adapted to more continuous tone parts of the image.
Moreover, the resolution conversion process could make further distinctions in the various aspects of the image beyond a simple division into pictorial vs. text and line art. Such distinctions might include distinguishing between low and high frequency halftone content, or between pictorial and graphic arts kinds of images.
Systems that implement resolution conversion processes, like those outlined above, show significant improvement in image quality compared to systems that do not. However, such systems are subject to problems.
One such problem is the need to somehow distinguish those parts of the image that have edges from those parts of the image that do not. Various processes have been proposed to identify such regions and to develop an image parallel to that being reproduced, a tag image that identifies the different characteristics of each part of the image.
Therefore, it is desirable to implement a method of reconstructing a contone image from its halftoned counterpart that is efficient in use of image path resources and at the same time is adaptive to the different characteristics of the underlying image so as to maximize the image quality of the output.